[DOCS] Added Data Frames subsection to ML section.

docs/en/stack/ml/dataframes.asciidoc

@@ -0,0 +1,29 @@
+[[ml-dataframes]]
+=== Data frames
+++++
+<titleabbrev>Dataframes</titleabbrev>
+++++
+
+A {dataframe} is a transformation of a dataset by certain rules defined during
+the creation of the {dataframe}. You can think of a it like a 
+spreadsheet or a data table.
+
+{es} datasets consist of individual documents that have fields and
+values in each field. This architecture makes hard to run analyses that require 
+reorganized or summarized fields of the dataset. {ml-cap} analyses need clean 
+and transformed data. That is the point where {dataframe}s come into play.
+
+To transform the data into a {dataframe}, you need to define a pivot. During
+pivoting, you create a set of features that transform the dataset into a
+different, more digestible format to make calculations on your data. Pivoting
+results in a summary of your dataset (which is a {dataframe}).
+
+When you define a pivot, you select one or more fields that your dataset will 
+be grouped by. You can select *categorical fields* (fields contain strings as
+values) for grouping. You can select *numerical fields* for grouping only as 
+histograms.
+
+examlpe - to do
+
+IMPORTANT: Creating a {dataframe} leaves your dataset intact. It does not amend
+the dataset itself.

docs/en/stack/ml/overview.asciidoc

@@ -11,3 +11,4 @@ include::buckets.asciidoc[]
 include::calendars.asciidoc[]
 include::rules.asciidoc[]
 include::architecture.asciidoc[]
+include::dataframes.asciidoc[]